Progress in the diagnosis and treatment of disorders of central visual mechanisms critically depends upon an understanding of the fundamental principles of organization of the visual cortex. The long-range goal of this project is to determine how cortical mechanisms of vision are controlled via thalamic inputs. In the cat, three separate channels of information originate in the eye and are distributed in unique combinations across the several layers of the thalamic relay complex composed of the lateral geniculate and medial interlaminar nuclei (LGN-MIN). The layers of the LGN-MIN distribute their outputs to striate and several areas of extrastriate cortex. The proposed research is designed to: (i) elucidate the functional relationships between these layers and their cortical targets; (ii) describe interactions among the LGN-MIN layers and corticocortical pathways; (iii) determine the dependence of response characteristics on the cortical column's interlaminar circuitry. For these experiments the following tools will be used, individually and in combination: (i) reversible inactivation of individual LGN-MIN subdivisions by direct injection of blocking agents; (ii) reversible inactivation of supragranular layers of area 17 by surface cooling; (iii) reversible inactivation of areas 17, 18 or 19 by cooling. A clue to the MIN's contribution to vision is evidence that it has a specific role in scotopic vision. The hypothesis that the MIN is more sensitive under dim-light than the LGN will be tested directly.